Does anyone know the logic or reasoning behind this? When I start the car when it's been baking in the sun or there is otherwise a large difference between the outside and inside air it seems the climate control sets the air to recirculate. AC only seems to come on in the auto mode when the temp is set to the lowest position.

sbiddle: A/C is more efficient using air from within the car rather than outside. Most cars default to recirculate when you turn the A/C on.

isn't the recommendation to open your window when driving around town and to use the ac with windows up when on the open road? it doesn't explain with the car would recirculate the air, especially when the AC isn't on and the car isn't moving

also recirculating the air eventually just makes the windows fog up when it's cooler outside

If the car is scorching hot when I get in then I open the window for a few minutes to let out the excess heat, then roll up the windows and operate the air conditioning. As others have mentioned, recirculate will allow the AC to bring the cabin temperature down to a comfortable level more quickly but make sure you switch back to fresh air to avoid fatigue if you are on a long trip.

richms: And let smelly diesel fumes in? No thanks. Recirc all the time except when the car is really hot from being parked.

Aircon will remove the moisture from the air, so fogging up shouldnt ever be a problem.

Agree with this.. On a hot day after car has been parked up i open windows and sunroof even just for a couple of minutes which gives the AC a fighting chance to bring temp down fairly quickly, by the time I'm out of the car park, windows back up, car almost pleasant, mind you I have tinted windows which definitely help the cause given black leather interior

To cool the car quickly you want the air coming out of the aircon to be as cool as possible. To do that you want the air entering the aircon to be as cool as possible. So if it's cooler outside it's better to use outside air. When it's hotter outside it's more efficient to use re-circulation.

(Slight simplification because the moisture content comes into it as well, but it doesn't make much difference.)

All climate control A/C (usually the Auto button/Option in newer cars) will default to Recirc when first turned on/activated.Mine pulls down the temp pretty quickly (Mazda3), then switches over to fresh air (usually after about 3 minutes).If it doesn't switch to fresh air, then there is something wrong.

I have read (but can't find again) a report on research undertaken in Europe where the authors concluded that people are more like to crash if they have their A/C set to recirc. The author's believed that reduced oxygen levels affected concentration and exacerbated fatigue. They recommended that A/C be designed to switch to fresh air after 15 minutes (or something like that) on recirc.

Mine stays on outside air unless the air quality sensor detects low quality air, then it changes to recirculate

From the BMW Website:

Automatic Air Recirculation (AAC) delivers noticeably better air quality in the cabin of your BMW. The system recognises pollutants and contaminants in the outside air and removes them in a microfilter. Should the concentration of pollutants exceed a maximum level, the system automatically closes off air intake from outside and temporarily recirculates the air in the cabin. The condensation sensor ensures that the windows of the vehicle do not become misted, even if humidity levels increase. In addition, the solar sensor measures the intensity and angle of the sun's rays, and the system automatically adjusts the air conditioner's performance to ensure a steady temperature is maintained. You and your passengers enjoy a steady cabin temperature, a clear view and excellent air quality at all times.

scuwp: Because it's easier to cool a carload of air faster than to cool all the air in the atmosphere. Initially it seems like a bad idea but it really does work once the initial cooling process starts.

this would make sense if a) the air con also came on with the recirculate function and b) if if was efficient to use ac, should I turn it on rather than open the windows. But generally it doesn't